The invention proceeds from a system for hydraulically adjusting the transmission ratio of a continuously variable transmission in a motor vehicle.
Continuously variable vehicle transmissions are, for example, known from DE 196 49 483 A or U.S. Pat. Nos. 5,337,628 and 5,431,602. As will be explained in detail in the following, the adjustment of the transmission ratio takes place hydraulically via the adjustment of a so-called primary valve. To adjust the transmission ratio, an electric current is applied to the primary valve which, for example, can be configured as a magnetic valve and, in this way, a specific hydraulic oil pressure at the transmission is adjusted. Such a valve, in general, has several switching stages which can be adjusted by the precise dimensioning of the drive current. A position of the valve, in general the center position, serves for the purpose of permitting no hydraulic oil through the valve (block position).
In the control of the valve, it is very important that the dimensioning of the drive current during vehicle operation can take place with good accuracy essentially independent of tolerances of the components of the valve and/or independent of the deterioration of the components of the valve.
The task of the present invention is to achieve a very precise dimensioning of this drive current in normal operation of the vehicle.
As already mentioned, the invention proceeds from a system to hydraulically adjust the transmission ratio of a continuously variable transmission in a motor vehicle. The adjustment of the transmission ratio takes place via the drive of at least one valve by an electrical signal. Furthermore, the transmission ratio is maintained essentially constant in one operating state in that the electrical signal is set to a pregivable value. For example, the valve then assumes the above-mentioned center position (block position). This operating state is also characterized as a ratio-hold state.
The essence of the invention is that the pregivable value can be changed in normal operation of the vehicle. The invention affords the advantage that component tolerances are compensated in the continuous operation of the vehicle or of the transmission. In this way, the adjustment complexity during the assembly of the vehicle is reduced. In addition, with the invention, the deterioration of components and especially the change of the spring in the valve (for example, setting of the spring) is compensated during the operation.
According to the invention, it is especially provided that the change of the pregivable value takes place in dependence upon an adaptation and the adaptation is carried out in the presence of pregivable operating conditions. According to the invention, the valve current, which is intended to lead to an adjustment of the center position of the valve, is adapted to the instantaneously present conditions (for example, deterioration, tolerances) during operation.
Furthermore, it can be provided that the pregivable value comprises a (fixed) base value and an offset value. The offset value is then changed or adapted in accordance with the invention. Here, it is especially provided that the offset value is applied not only to adjust the above-mentioned center position of the valve but is considered also generally in the formation of the electrical signal.
In a first variation of the invention, the pregivable value, especially the offset value, is changed when a change of the transmission ratio is detected for the presence of the pregivable operating conditions (ratio-hold state).
In this connection, it is especially provided that at least one of the pregivable operating conditions is present when the transmission is in an operating state wherein the transmission ratio is held constant especially by setting the electrical signal to the pregivable value (ratio-hold state). In the special case of the already described valve, this means that the valve is driven to adjust the center position (block position) whereupon the transmission ratio should ideally not change or should change only to a slight extent. If the transmission ratio, however, changes significantly, then an adaptation of the drive current is required. It is especially provided that the pregivable value, especially the offset value, is changed in such a manner that the change of the transmission ratio is countered.
In addition to the operating condition mentioned in the last section, other ones of the pregivable operating conditions can then be present:
when quasi steady-state operating conditions of the transmission are present and it is especially provided that quasi steady-state operating conditions are present when the vehicle longitudinal speed (especially the output rpm of the transmission) does not change significantly as a function of time; and/or,
when the vehicle longitudinal speed, especially the output rpm or the input rpm of the transmission is within a pregivable range; and/or,
when the temperature of the transmission is within a pregivable range; and/or,
when the transmission ratio is within a pregivable range.
In a second variation of the invention, it is provided that at least one of the pregivable operating conditions is present when the transmission is in an operating state in which the transmission ratio is essentially constant or does not significantly change as a function of time.
While the first variation undertakes the adaptation during the operating state of the transmission wherein the electrical signal is set to a pregivable value, the second variation proceeds from an operating state wherein the transmission ratio is controlled to be constant. If one first does not consider the minimum and maximum transmission ratios, then the valve must be in its center position when the constant transmission ratio is present. This can be used for adaptation in that the pregivable value, especially the offset value, is changed in dependence upon the electrical signal which is adjusted in the presence of the pregivable operating conditions.
In addition to the operating condition mentioned in the last section, other ones of the pregivable operating conditions can be present:
when the transmission ratio is within a pregivable range;
and/or,
when the time-dependent change of the transmission ratio of the transmission is within a pregivable range; and/or,
when quasi steady-state operating conditions of the transmission are present and it is especially provided that quasi steady-state operating states are present when the vehicle longitudinal speed (especially the output rpm of the transmission) does not significantly change as a function of time; and/or,
when the temperature of the transmission is within a pregivable range.
A third variation of the invention provides that a pressure is controlled via the drive of the valve and a pressure value is detected which represents the pressure in the ratio-hold operating state of the transmission. The pregivable value can then be changed in dependence upon the detected pressure value.
The idea of the third variation of the invention proceeds from the fact that a pressure sensor is present for detecting the pressure controlled by the valve or this pressure can be computed from other quantities. The center position of the valve is so adjusted or adapted that a specific pressure adjusts.
Furthermore, it can be provided that the electrical signal, that is the current value, is adjusted during adaptation in such a manner that the adjustment of the transmission ratio to higher transmission ratios is undertaken, that is, the CVT is therefore shifted up. Here, the electrical signal adjusted during the adaptation can be less than the electrical signal adjusted before the adaptation by a difference value. That means that the current, which is adjusted during the adaptation, is equal to the ratio-hold current value less a difference value; that is, the current during the adaptation is somewhat less than the ratio-hold current.